The data is transmitted in data packets or in transmission channels with a fixed bandwidth in the data transmission network. The data packets are usually of a permanently predefined size. Information relating to the destination is specified in a packet header. The user information data to be transmitted is located in a packet body.
During the transmission in transmission channels, in the simplest case a line is used for each user information connection. However, a plurality of user information connections are usually transmitted over one transmission link, fixed time windows being predefined in one time frame for the transmission of the user information data of a user information channel.
As a result of opening the telecommunications market, there are network operators which do not have their own transmission links or switching devices. FIG. 1 shows a telecommunications network 10 whose operator has leased the greater part of the switching and transmission equipment from a conventional network operator and which is therefore referred to below as a new operator. The telecommunications network 10 contains three local switching offices 12 to 16 which are associated with the conventional network operator and are used by the new operator. The network operator of the telecommunications network 10 also has leased transmission links 20 to 26 from the conventional network operator. The transmission link 20 connects the local switching office 12 to a subscriber line concentrator 30 to which a multiplicity of subscribers are connected, two subscribers Tln1 and Tln2 of which are shown in FIG. 1. A telecommunications system 32, which is used by a private company for toll-free switching of connections within the company's premises being also connected to the concentrator 30. The concentrator 30 has also been leased by the conventional network operator. A PCM30 (pulse code modulation) system, for example, is used for transmission on the transmission link 20.
The transmission link 22 connects the local switching office 14 to a subscriber line concentrator 34 to which a multiplicity of subscribers are also connected, subscribers Tln3 and Tln4 of which are illustrated in FIG. 1. A telecommunications system 36 is connected directly to the local switching office 14 via a transmission link 38.
The transmission link 24 lies between the local switching offices 12 and 14. The local switching offices 14 and 16 are connected using the transmission link 26. A telecommunications system 40 is connected to the local switching office 16 via the public telecommunications network 42. Voice data according to the ISDN (Integrated Services Digital Network) standard are transmitted in the public telecommunications network 42.
The network operator of the telecommunications network 10 attempts to utilize the leased transmission links 20 to 26 as well as possible. Compression algorithms for voice are defined, for example, in the standards of the ITU-T (International Telecommunication Union—Telecommunication), for example in the Standards G.723 to G.729. According to the standards, the voice information to be switched within the scope of a normal voice connection can be reduced to bit rates of up to 8 kbit/s on certain links or sections without a disruptive loss of voice quality. This permits a basic channel with a bit rate of 64 kbit/s to be used for transmitting up to eight voice connections. If facsimile data, program data or file data are transmitted over a connection, suitable measures can also be taken to reduce the bandwidth.
The voice quality of a connection depends in particular on the following factors:                falsification due to voice processing, for example due to voice compression/decompression,        delays due to transmission, packetizing and/or voice processing,        echo effects, and        information losses, for example due to transmission errors or due to losses of packets or cells.        
Multiple transcoding leads to audible degradations in the voice quality, for example to:                voice noise and relatively large voice falsifications,        reduction in the clarity and comprehensibility,        the increased difficulty in recognizing the speaker by his voice, and        listener and speaker echo owing to the relatively long delay times in the transmission of transcoded voice.        
Multiple transcodings are therefore to be avoided as much as possible.
However, multiple transcodings occur to an increasing degree due to the following causes:                connections are switched between different networks,        the use of reduction methods in private networks, and        the inclusion of new techniques in voice transmission, for example voice transmission over the Internet.        